1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to compositions of semiconductor layers of a semiconductor laser.
2. Description of the Related Art
As-a-1060 nm-band semiconductor laser completely free from aluminum, there has been reported a semiconductor laser of a strain compensation structure comprising an n-InGaAsP clad layer, an undoped InGaAsP optical waveguide layer, an InGaAsP barrier layer which is 0.7% in tensile strain, an InGaAs quantum-well active layer which is 2.1% in compression strain, an InGaAsP barrier layer which is 0.7% in tensile strain, an undoped InGaAsP optical waveguide layer, a p-InGaAsP clad layer and a p-GaAs capping layer formed on an n-GaAs substrate in this order. See, for instance, "Applied Physics Letters, 99(1996)" pp248. However the Al-free semiconductor laser has reliability of only about 250 mW class and practically cannot be used as a higher output semiconductor laser.
Further as a 680 nm-band semiconductor laser whose active layer is free from aluminum, there has been reported a semiconductor laser in which a GaInP active layer is imparted with compressive strain, a side barrier layer is provided with an AlGaInP layer having tensile strain sufficient to cancel the compressive strain of the active layer, and the end face band gap is increased by relaxation in crystal structure in the vicinity of the radiating end face of the laser, thereby reducing absorption of light during oscillation and suppressing deterioration of the end faces due to absorption of light. However in order to use the tensile-strained side barrier layer in a 1000 nm-band semiconductor laser, an InGaAs active layer which is high in proportion of In must be used and the thickness of the active layer must be as small as the critical film thickness, which makes the crystal unstable. Further it is difficult to obtain a high quality crystal due to diffusion of In and accordingly it is difficult to obtain a semiconductor laser of high reliability.